1. Field of the Invention
This invention relates to a device for fluidly tensioning a bolt to enable its nut to be tightened or loosened.
2. Prior Art
Heavily axially loaded bolts are usually axially pre-tensioned so that the threaded joint under load will normally keep the structural sections thereby bound together in an unaltered position without any further elongation of the bolts. It is not usually possible to carry out pre-tensioning or pre-stressing of bolts just by tightening of a nut, due to the uncertainty of accomplishing the desired pre-tension with sufficient accuracy in this way. For example, too large a torque may be applied to a long bolt, or there can be difficutly in general in attempting to apply the necessary torque to the nut. In certain constructions having a number of such bolts, there is the requirement that the pre-tension in each bolt is to be accurately controlled, and in many instances to be evenly distributed between the various bolts. The latter requirement is prescribed, for example, for bolts in rolling mills, larger diesel engines, reactor vessels and pressure pipe lines.
There are known devices for pre-tensioning screwed bolts with the help of hydraulically operated pistons. In one form of such device, the device is removed after the bolt is tensioned and needs to be reattached for releasing the nut. Such device has the disadvantage of large dimensions and comparatively high weight. In that it must be moved from one connection to the next for each tightening procedure, it must be handled using special suspending devices because of its weight. Such device is therefore difficult to maneuver, slow in use, and cannot be used to advantage when several bolts are to be tightened simultaneously.
Another device uses an auxiliary nut and likewise the auxiliary device for tensioning the bolt must be applied to the bolt which is to be tensioned and must be removed afterwards.
A further known device has the disadvantage that the load in the finally tensioned bolt is transferred through two threaded joints, a prerequisite of the device being accurate guidance of a piston along two pairs of cylindrical cooperating guide surfaces. Further, this device does not compensate for the effect of axial deviations in the direction of the bolt in relation to the seat when pressure has been released. A screwed bolt which from the beginning shows such an axial deviation tends while being elongated to be straightened in the direction of force, which happens under comparatively large contact pressure between force transmitting surfaces. It may also be that the bolt will be subjected to bending. Due to the construction of such device, release of the fastening nut in the conventional manner in a emergency, involves considerable difficulty.